A secondary battery, which is easily applied to various product groups and has electrical characteristics such as high energy density, is universally applied not only for a portable device but also for an electric vehicle (EV) or a hybrid electric vehicles (HEV), an energy storage system or the like, which is driven by an electric driving source. The secondary battery is attracting attention as a new environment-friendly energy source for improving energy efficiency since it gives a primary advantage of reducing the use of fossil fuels and also does not generate by-products by the use of energy at all.
Secondary batteries widely used at the preset include lithium ion batteries, lithium polymer batteries, nickel cadmium batteries, nickel hydrogen batteries, nickel zinc batteries and the like. An operating voltage of the unit secondary battery cell, namely a unit battery cell, is about 2.5V to 4.2V. Therefore, if a higher output voltage is required, a plurality of battery cells may be connected in series to configure a battery pack. In addition, depending on the charge/discharge capacity required for the battery pack, a plurality of battery cells may be connected in parallel to configure a battery pack. Thus, the number of battery cells included in the battery pack may be variously set according to the required output voltage or the demanded charge/discharge capacity.
A battery cell serving as the secondary battery, in particular, a battery cell serving as a lithium secondary battery, generally includes an electrode assembly composed of a positive electrode containing a lithium transition metal oxide as an electrode active material, a negative electrode containing a carbon-based active material, and a separator, into which a lithium electrolyte is impregnated.
The electrode of the battery cell is fabricated by coating an electrode foil with electrode slurry. The electrode slurry is prepared by mixing an electrode mixture composed of an electrode active material, a conductive material and a binder for bonding these materials to an electrode foil in an organic solvent. Here, the positive electrode active material mainly employs lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide and lithium composite oxide, and carbonaceous material is mainly used as the negative electrode active material.
In addition, the electrode of the battery cell is generally formed by dispersing an active material, a conductive material and a binder in a solvent to prepare slurry, and then applying the slurry directly to the current collector, or applying the slurry to an upper portion of a separate support and then laminating a film peeled off from the support onto the current collector. After that, the electrode is rolled into a roll shape, and then residual solvent and residual moisture are removed by means of an electrode drying device.
A conventional electrode drying device generally includes a drying chamber in a vacuum atmosphere and a heat source for supplying dry hot air from the outside of the rolled-shaped electrode rolled in the drying chamber.
However, in the conventional electrode drying device, a temperature difference is generated between the outside of the roll-shaped electrode, which is dried by indirect dry hot air, and the inside of the electrode. Accordingly, in the conventional electrode drying device, it is difficult to secure the drying uniformity between the outside of the electrode and the inside of the electrode, and thus residual solvent and residual moisture are not efficiently removed at the inside of the electrode. In particular, in the case of a negative electrode, a spring back phenomenon causing the swelling of the electrode appears. The spring back phenomenon may lower the energy density of the battery cell or cause stability problems later.
Therefore, it is required to search for a method of increasing the drying efficiency of the electrode so as to solve the above problems of the electrode drying device.